const s1 = Symbol('xxx');

console.log(s1);
console.log(typeof s1); //symbol
console.log(Symbol.prototype.valueOf.call(s1)); //Symbol(xxx)
console.log(s1.valueOf()); //Symbol(xxx) //← 返回的是s1本身
console.log(s1.valueOf() === s1); //true
console.log(s1.valueOf() === 'Symbol(xxx)'); //false

const s2 = Object(Symbol.prototype.valueOf.call(s1));
console.log(s2); //[Symbol: Symbol(xxx)]
console.log(typeof s2); //object


console.log(typeof (function abc() {})); //function
